1. Field of the Invention
The present invention relates to a rolling mill and, more particularly, a work roll crossing type sheet rolling mill which exhibits an excellent ability with which it controls the crown of materials to be rolled, as well as a hot rolling system, a rolling method and a rolling mill revamping method.
2. Description of the Prior Art
In roll cross type four high rolling mills which are available on the market, a roll pair consisting of an upper work roll and an upper back-up roll and a roll pair consisting of a lower work roll and a lower back-up roll are moved such that the axes of the two roll pairs cross each other on a horizontal plane. Such roll cross type four high rolling mills have been described in, for example, Mitsubishi Heavy Industrial Co., Ltd. Technical Report, Vol. 21, No. 6 (1984) from p61 to p67.
Four high rolling mills in which crossing of only the work rolls is performed have been proposed earlier than the pair cross type rolling mills in, for example, Japanese Patent Unexamined Publication No. 47-27159. However, it is only the aforementioned pair cross type rolling mill used for hot-rolling that has been put into practical use.
In the so-called pair cross type rolling mill in which a pair of the work roll and the back-up roll cross another pair of the work roll and the back-up roll, although generation of slip or thrust between the back-up roll and the work roll is suppressed, since the center of a metal chock of the back-up roll which is directly subjected to a rolling load shifts from the center of a reduction screw, rotational moment is exerted to the metal chock, generating a local load to the mill stand. Consequently, smooth rolling operation is prohibited and wear of the metal chock is accelerated. To prevent these drawbacks, a very rigid beam may be provided to balance the drive side and the operated side of the rolling mill. However, provision of such a rigid beam increases the overall size of the rolling mill.
Whereas the thrust exerted to the work rolls which do not cross each other is generally 1 to 2% of the rolling loads in the case of the hot rolling, the thrust exerted to the work rolls which cross each other is 5% of the rolling load, which is twice or three times that of the work rolls which do not cross each other.
Adjustment of the cross angle during rolling is necessary, because it enables changes in the rolling load or the crown of the material to be rolled to be coped with during rolling or because it enables the incorrectly set cross angle to be corrected. These would not be achieved by the bender alone, and changes in the cross angle in a state wherein a large rolling load is being exerted are thus necessitated. In recent years, the rolling operation in the finish rolling mill stand of the hot strip mill has been directed to continuous operation. In the continuous rolling operation, the metal chock of the back-up roll must be moved during rolling, i.e., under enormous rolling loads, thus necessitating a special bearing. This makes the structure of the rolling mill more complicated. Also, a troublesome maintenance is necessary due to scales entering the lower portion of the rolling mill stand, and productivity is greatly reduced.
Thus, crossing of only the work rolls enables accurate changes in the cross angle to be readily performed during rolling without making the structure complicated. The force required for the crossing of the work rolls is very small and is about 1-2% of the rolling loads.
The rolling mill in which crossing of only the work rolls is performed cannot be put into practical use for the following two reasons.
First, when the work rolls cross the back-up rolls, an enormous amount of thrust is exerted to both the work rolls and the back-up rolls in two directions along the axis of the rolls, as described in "Research of Machines, Vol. 42, No. 10 (1990)" from page 71 to page 72. This thrust, which changes depending on the cross angle, is about 30% of the rolling loads. The thrust bearing of the large diameter back-up roll may sustain this thrust. However, it is very difficult for the work roll whose diameter is one half or less than that of the back-up roll to do that.
The second reason is roll wear caused by relative slip between the back-up roll and the work roll. Since the work rolls are changed with new ones every 2 or 3 hours due to wear caused by the material to be rolled which is greater than wear caused by relative slip, changing of the work rolls causes no problem. However, changing of the back-up roll takes place every 10 or 20 days and requires a long time. Therefore, frequent changing of the back-up rolls due to rapid wear greatly reduces productivity.